1. Field of the Invention
The present invention relates to electrical connectors and more particularly to insulation displacement contact terminals.
2. Brief Description of Prior Developments
In order to further miniaturize various electronic systems, insulation displacement contact terminals have been substituted for soldered connections in a number of applications. Such terminals are disclosed, for example, in U.S. Pat. Nos. 4,050,760 and 4,385,794. In such terminals, insulated wires to be connected are inserted into contact channels having opposed transverse projections known as dimples. These dimples remove insulation from the wires so inserted to allow electrical connection between these wires and the terminal. Heretofore these contact dimples have been formed by a process of inwardly shearing the side walls of the contact channels.
The effectiveness of the connection with those contact dimples is dependent, at least in part, on the amount of pressure applied to connected wires by the contact dimples. A continuing need, therefore, exists for means by which pressure applied by such dimples on the connecting wire can be increased.
It has been found that the amount of pressure which may be applied to inserted wires is advantageously affected by a number of factors including the stiffness or spring rate of the contact channel, the channel yield strength and the sharpness of the front face of the dimples. It has also been found that the shearing process for forming these dimples may adversely affect these factors. In the method of the present invention the contact dimples are formed in a compressive operation in which a compressive force is inwardly exerted on a metal blank after which the metal is formed into a contact channel. For the purpose of this disclosure a compressive operation will be considered to be any metal forming operation including sizing, swaging, coining and extruding in which a metal blank or slug is squeezed to thereby change its form through the direct application of compressive force. The metal strained in this way by compressive stresses is plastically deformed and behaves like a viscous liquid. Preferably the method of the present invention will be carried out by swaging and preferably in a series of successive steps.
In the present invention insulation displacement contact dimples are preferably produced in a punch press in three general steps. In the first step, a metal strip stock element is positioned between a first concave upper die and a first convex lower die. In this step the metal is not only stretched, but is swaged along the side of the dimple shaped element. An upper cavity is formed between the dimple shaped element and the first upper die and the metal is extruded upwardly toward that upper cavity. In the second step, the dimple shaped element is positioned between a second concave upper die and a second convex lower die. This lower die has a radius that is smaller than the radius of the first convex lower die used in the first step. Thus, the height of the dimple is raised. In this second step swaging also occurs on the side of the dimple but at a greater height than on the first step. In a third step, the dimple shaped element is positioned between still another third concave upper die and a third convex lower die. This third convex lower die has a greater radius and a steeper slope than the second convex lower die. In this step a lower cavity is initially formed between the dimple shaped element and the third convex die and an upper cavity between the dimple shaped element and the third concave die. The dies press against the dimple shaped element at points between these upper and lower cavities and begin to swage the metal. The forces involved are such that the metal will flow into the upper cavity first and then once the upper cavity is filled will flow into the lower cavity. The two cavities are needed since the metal at the top and bottom of the dimple shaped element will be thinner than the metal in the middle. The lower cavity allows the extra metal in the middle to flow into it while the upper cavity is still being filled near the top and bottom of the dimple. The process is also capable of flowing the metal into the upper die into a radius that is smaller than the thickness of metal. Alternatively, the third step may involve filling the lower end of the dimple shaped element by thinning the metal at the lower end and extruding the metal upwardly. The method produces a sharp dimple with a small radius on the front face that efficiently pierces wire insulation and extrudes into the copper conductor. In many cases the first, second and third upper dies will be identical and the same upper die can be used for all three steps.